Reservation multiple access

ABSTRACT

A mobile station accesses a base station by randomly selecting a first reverse link common control channel from a set of random access channels. The mobile station transmits a request portion of an access probe over the first reverse link common control channel. The request portion is subject to collision with other signals. The request portion comprises a hash identification which is derived from a uniquely identifying number using a hash function. The hash identification quasi-uniquely identifies the mobile station. The mobile station receives a channel assignment message from the base station designating the hash identification and a reserved access channel. The reserved access channel provides communication with a low probability of contention. The mobile station transmits a message portion of the access probe over the reserved access channel.

CLAIM OF PRIORITY UNDER 35 U.S.C. §120

[0001] The present application for patent is a Continuation and claimspriority to patent application Ser. No. 09/275,729 entitled “ReservationMultiple Access” filed Mar. 24, 1999, now allowed, which claims priorityto U.S. patent application entitled, “Reservation Multiple Access”, Ser.No. 09/173,572, which was filed on Oct. 15, 1998, both of which areassigned to the assignee hereof and hereby expressly incorporated byreference herein.

BACKGROUND OF THE INVENTION

[0002] I. Field of the Invention

[0003] The invention relates generally to wireless communications. Moreparticularly, the invention relates to multiple access in a wirelesscommunication system.

[0004] II. Description of the Related Art

[0005] In a typical wireless communication system, a plurality of mobilestations communicate through a common base station. Because the basestation has finite resources available, the mobile stations compete foraccess to the base station resources. FIG. 1 is a block diagram showinga typical modern wireless communication system 10. The system iscomprised of a series of base stations 14. A set of mobile stations 12communicate with the base stations 14. The mobile stations 12communicate with the base stations 14 over a forward link channel 18 anda reverse link channel 20. As used herein, the term “channel” refers toboth a single communication link between the base station and a specificmobile station as well as a grouping of communication links, typicallyhaving a common function. FIG. 1 shows a variety of types of mobilestations. For example, FIG. 1 shows a hand-held portable telephone, avehicle mounted mobile telephone and a fixed location wireless localloop telephone. Such systems offer voice and data services. Other moderncommunication systems operate over wireless satellite links rather thenthrough terrestrial base stations.

[0006] An industry standard for a wireless system using code divisionmultiple access (CDMA) is set forth in the TIA/EIA Interim Standardentitled “Mobile Station—Base Station Compatibility Standard forDual-Mode Wideband Spread Spectrum Cellular System”, TIA/EIA/IS-95, andits progeny (collectively referred to here in as IS-95), the contents ofwhich are also incorporated herein by reference. Among other channels,IS-95 defines a reverse link random access channel which is used by themobile stations to communicate with a base station. The access channelis used for short signaling message exchanges such as call originations,responses to pages and registrations. For example, for prolongedbi-directional communications, a dedicated forward link and reverse linktraffic channel pair are established between the mobile station and thebase station. The access channel can be used to transfer informationfrom the mobile station to the base station before the traffic channelis established in order to facilitate establishment.

[0007] The access channel defined by IS-95 is a random access channelmeaning that a mobile station randomly chooses a portion of the accesschannel resources over which to transmit an access probe. Due to therandom nature of the access channel, there is no guarantee that only asingle mobile station will attempt access on the chosen portion.Therefore, when an access probe is sent, it may fail to be received bythe base station for one of several reasons. It may fail because thepower level received at the base station is too low compared to thecurrent interference levels. It may fail because another mobile stationattempts to use the same portion of the access channel resources at thesame time causing a collision. In any case, when the access probe is notreceived at the base station, the mobile station randomly selectsanother portion of the access channel resources and attempts access tothe system, perhaps using a higher signal level. In order to avoid aseries of lockstep failures between two mobile stations after an initialcollision, the retransmission process is also randomized.

[0008] In order to select a portion of the access channel resources,according to IS-95, the mobile station randomly selects one of a set ofone or more access channels defined by CDMA techniques. Once an accesschannel is selected, the mobile station is constrained to begintransmission of the access probe at one of a set of re-occurring slotboundaries. The mobile station randomly selects a slot boundary andbegins transmission. Such operation is referred to as slotted alohaoperation and is well known in the art.

[0009] One key aspect of a random access system is load control. Loadcontrol is used to statistically control the rate at which access probesare received at the base station. Load control in a slotted aloha systemis important because as the number of access attempts increases, thenumber of collisions also increases. As the loading further increases,the number of successful access attempts actually begins to fall due tothe system resources being consumed with collisions. Therefore, in aslotted aloha system, it is advantageous to keep system loading at lessthan 18% of the fully loaded capacity, otherwise unstable behavior canresult.

[0010] Loading is also a function of the amount of interference in thesystem. The available capacity of a system decreases as the interferenceincreases. As the load on the random access channel increases, it maycause significant interference to other channels in the system such asthe traffic channels. According to IS-95, loading on the access channelis controlled by the insertion of random delay (called access probeback-off) between a failed access attempt and a follow up attempt.However, IS-95 lacks any mechanism for quickly enabling and disablingaccess to the access channel in order to control loading.

[0011] According to IS-95, when a mobile station sends an access probe,it transmits a uniquely identifying number such as the electronic serialnumber (ESN) of the mobile station along with other information in apreamble. In addition, the access probe comprises a message whichspecifies the purpose of the probe or carries user data. For example,the message may designate a telephone number for use in a callorigination. An access probe is typically between 80 and 150milliseconds (msec) in duration.

[0012] According to IS-95, the mobile station initially transmits theaccess probe at a first level. If the base station does not respond withan acknowledgment after a predetermined amount of time, the mobilestation continues to repeat the access probe at increasingly higherpower levels.

[0013] This method of access does not yield a very efficient use ofsystem resources. First, the access probe is fairly lengthy and themobile station continues to transmit the entire access probe even if thebase station is unable to receive the access probe, thus, spewingun-useful energy into the system, wastefully expending mobile stationresources and reducing system capacity. According to IS-95, once themobile station has begun to transmit, no power control mechanism existsby which the base station can increase or decrease the transmit power.If the reverse link is subjected to a deep fade, the transmission mayfail and the mobile station retransmits the message at a higher powerlevel which may not be necessary in the absence of the fade. The basestation has no means to request more power during the deep fade nor torequest a reduction in power during the subsequent retransmission. Inaddition to consuming significant system resources, the access methodaccording to IS-95 can stretch to cover a significant amount of timeadding delay to the system. According to IS-95, data is transmitted overthe access channel at only one data rate regardless of the amount ofdata or the quality of the connection between the mobile station and thebase station.

[0014] Thus, there has been a need in the art to develop a multipleaccess system which introduces less delay and makes more efficient useof the available system resources.

SUMMARY OF THE INVENTION

[0015] Reservation multiple access (RsMA) is used to provide multipleaccess to a plurality of mobile stations. The access probes used toaccess the system are divided into two different portions: a requestportion and a message portion. The request portion comprises a numberwhich “quasi-uniquely” identifies the mobile station. For example, ahash identification can be derived from a longer number which uniquelyidentifies the mobile station using a hash function. The request portionalso comprises a preamble to facilitate detection. The length of therequest portion is small in comparison to the length of the messageportion.

[0016] The request portion is sent over a random access channel. Forexample, in one embodiment, the request portion is transmitted overslotted aloha channel in which the slot boundaries follow closely afterone another such as on the order of the length of several requestportions.

[0017] If the request portion is properly detected by the base stationand if resources are available, the base station assigns a reservedaccess channel using a channel assignment message. The channelassignment message comprises the hash identification. The mobile stationsends the message portion over the reserved access channel. The reservedaccess channel provides communication with a low probability ofcontention. In one embodiment, the message portion can comprise arequest for a traffic channel or other system administration message orit may contain a datagram of user information. In one embodiment, themessage portion can take on one of a set of variable data rates.

[0018] In another embodiment, a forward link channel sends power controlinformation to the mobile station while it is transmitting over thereserved access channel. In yet another embodiment, the channelassignment messages, the power control information, or both, are sentfrom a plurality of sectors, base stations or both.

[0019] In one embodiment, a base station send can send a wait message toa specific mobile station or to a class of mobile stations over theforward link channel assignment channel which also carries the channelassignment messages. The wait message delays subsequent access attemptsby the subject mobile stations. In another embodiment, a wait messagecan be used to quickly disable access to the system in order to controlloading.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] The features, objects and advantages of the present inventionwill become more apparent from the detailed description set forth belowwhen taken in conjunction with the drawings in which like referencecharacters identify correspondingly throughout and wherein:

[0021]FIG. 1 is a block diagram showing a typical modern wirelesscommunication system.

[0022]FIGS. 2A and 2B are flow charts showing mobile station operationin RsMA.

[0023]FIG. 3 is a representational diagram showing a series of channelsin a RsMA system.

[0024]FIG. 4 is a representational diagram showing an exemplifying datastructure for the forward power control common channel.

[0025]FIG. 5 is graph illustrating mobile station transmission power ina closed loop system.

[0026]FIG. 6 is a representational diagram showing the coverage areasectors of a multisectored base station.

[0027]FIG. 7 is a block diagram of the multisectored base station.

[0028]FIG. 8 is a block diagram of an exemplifying mobile stationarchitecture.

DETAILED DESCRIPTION OF THE INVENTION

[0029] In order to overcome the limitations of the prior art, theinvention uses a reservation multiple access (RsMA) format to facilitaterandom access to the system. In order to increase efficiencies, theaccess message is divided into two different portions: a request portionand a message portion. The request portion is sent over a random accesschannel. In response, a reserved access channel is assigned. The messageportion is sent over the reserved access channel. Through the use of areserved access channel, in one embodiment, closed loop power control isapplied to the message portion of the access probe. Together with otherfeatures, the invention lends efficiencies to the access process.

[0030] The invention is best understood by way of example. FIGS. 2A and2B are flow charts exemplifying mobile station operation in a RsMAsystem according to the invention. FIG. 3 is a representational diagramshowing a series of channels and messages in a RsMA system which can beused to facilitate an understanding of FIG. 2.

[0031] Referring to FIG. 2A, flow begins in start block 100. In block102, the sequence number and probe number are set to 0. In block 104,the mobile station randomly selects a forward link channel assignmentchannel (F-CACH) from a set of forward link channel assignment channelssupported by the system. For example, the mobile station selects then-th forward link channel assignment channel such as F-CACH(n) 200 shownin FIG. 3. In one embodiment, the number of forward link channelassignment channels is programmable and can be reduced to 1 or even 0 inorder to reduce the number of successful accesses.

[0032] In block 106, the mobile station estimates the signal quality ofthe pilot signal received from the corresponding base station. Forexample, the mobile station may estimate the ratio of the energy in thecarrier to the noise power density (Ec/Io) at which the pilot signal isreceived. Block 108 determines whether the pilot signal quality exceedsa predetermined threshold. If not, the mobile station assumes that theforward link channel has faded and flow continues back to block 106until the signal quality improves. Due to the rapid fading nature of theterrestrial channel, adverse fading conditions typically correctthemselves quite rapidly. By avoiding transmission during a deep fade,the mobile station can increase the likelihood that it will receive abase station response on the F-CACH as described in more detailed below.Blocks 106 and 108 are optional and some embodiments may not containthis feature.

[0033] If it is determined in block 108 that the signal quality of thepilot signal exceeds the threshold, flow moves to block 110 wherein themobile station randomly selects a reverse link common control channel(R-CCCH) corresponding to the selected R-CCCH. For example, the mobilestation selects the c-th reverse link common control channel, such asR-CCCH(c) 202 shown in FIG. 3. In one embodiment, the F-CACH isassociated with a plurality of R-CCCH. In block 112, the mobile stationinitializes the transmit power to an initial power level (IP). In oneembodiment, the value of IP is determined based upon the signal qualityof the pilot signal as well as other factors. In another embodiment, thevalue of IP is a fixed or programmable value. Flow continues through offpage connector 114 to off page connect 116 of FIG. 2B.

[0034] In block 118, the mobile station transmits a request portion ofan access probe comprising a preamble and hash ID over the R-CCCH(c)202, as shown by the request message 210. The hash ID is derived frominformation that is unique to the transmitting mobile station. Accordingto one of a plurality of well known techniques, the hash value isgenerated by a hash function that maps an input number comprising alarge number of bits into an output number that is shorter. For example,in one embodiment of the invention, the input information for the hashfunction comprises the electronic serial number (ESN) of the mobilestation which is, according to IS-95, a 32 bit number assigned by themobile station manufacture which uniquely identifies the mobile stationequipment. Using 32 bits, over four billion mobile stations can beassigned a unique ESN. The output of the hash function is, for example,a 12 bit number defining 4096 different “quasi-unique” hash ID values.Although not unique, the length of the hash ID is sufficient to make itextremely unlikely that more than one mobile station operating withinthe coverage area of a base station will generate the same hash ID andtransmit the request portion of an access probe at the same time. Theuse of this hash ID allows less information to be transmitted comparedto IS-95, while still distinguishing that mobile station from all othersin the area in the vast majority of cases. If a collision occurs betweentwo or more mobile station using the same hash ID at the same time, someor all of the access attempts may fail. In such a case, the unsuccessfulrequest portion is retransmitted again and the random back-off periodsreduces the risk of a subsequent collision.

[0035] Eventually, during the course of the access, the mobile stationmust be uniquely identified to the base station. However, such uniqueidentification is not necessary in order to proceed with system accessat this point. The use of a hash ID significantly reduces the amount ofdata which is transmitted in the request portion of the access probe.According to the invention, unique identification of the mobile stationis accomplished within the message portion of the access probe ratherthan in the request portion.

[0036] In block 120, the mobile station monitors the F-CACH(n) 200 todetermine whether the access probe is successfully decoded by the basestation. For example, in FIG. 3, in one scenario, the base stationresponds by transmission of a responsive message 212. The responsivemessage comprises the hash ID of the mobile station to which it isdirected. The responsive message also comprises a cyclic redundancycheck (CRC) value or other error detection mechanism. In one embodiment,the F-CACH(n) 200 is associated with a number of R-CCCH(c) and can carrymessages intended for a number of different mobile stations, each ofwhich includes a CRC value. In block 122, the mobile station monitorsresponsive messages carried on the F-CACH(n) and determines whether afailure is detected by reliance on the CRC. If a failure is detected,flow continues in block 126 as explained below. In one embodiment, thebase station retransmits a repeat responsive message 212′ if no responsefrom the mobile station is detected. In FIG. 3, the response message isrepeated D2 seconds after the end of the initial transmission such thatthe mobile station timer D1 does not expire until the end of therepeated response message 212′. In one embodiment, the mobile stationsoft combines energy from to original response message 212 and therepeat response message 212′ to improve performance according to wellknown techniques.

[0037] If no failure is detected in block 122, the process moves toblock 124 and determines whether the specified hash ID transmitted inthe responsive message 212 carried on the F-CACH(n) 200 matches the hashID transmitted by the mobile station. If the hash ID does not match orif a failure was decoded in block 122, flow continues to block 126.Block 126 determines whether the D1 timer has expired. The D1 is resetwhen the request portion of the access probe is transmitted andaccumulates time until it has timed out. For example, in FIG. 3, theperiod of the D1 timer is indicated by the double arrow line labeled D1,beginning from the end of the request portion 210 of the access probe.If the D1 timer has not expired, the mobile station continues to monitorthe F-CACH(n) 200 beginning in block 120.

[0038] If the hash ID matches, flow continues from block 124 to block146. Block 146 determines whether the responsive message 212 is a waitmessage. For example, the base station may send a wait message whichdirects the mobile station to attempt access again after the passage ofsome amount of time. In this way, the base station can control the basestation loading caused by the mobile stations using these reverse linkchannels. By setting the wait time to infinity, the system has amechanism for quickly disabling access to the access channel in order tocontrol loading. If the message is a wait message, flow continuesthrough off page connector 148 to off page connector 158 of FIG. 2A. Inblock 160, the mobile station generates a pseudo random number PN(b) tobe used for a back-off timer. In block 162, the mobile station waitsPN(b) slot times before re-entering flow to attempt another access. Inone embodiment, the wait message simply directs the mobile station toenter the routine which chooses the back-off period. In anotherembodiment, the base station can direct the mobile station to wait anadditional amount on top of the wait specified by the randomly chosennumber. In yet another embodiment, the base station can specify a factorby which the back-off period is multiplied in order to change the waitperiod.

[0039] Returning again to FIG. 2B, if no wait message is received inblock 146, flow continues to block 150. Block 150 determines whether achannel assignment message is received. If no channel assignment messageis received, flow continues to block 152 where access failure isdeclared and the mobile station enters a system determination state. Inother embodiments, other types of responsive messages are included inthe system and are detected before a failure is declared.

[0040] If a channel assignment message is detected in block 150, flowcontinues to block 154. The channel assignment message specifies areverse link, reserved access channel (R-RACH) for use by the mobilestation, such as R-RACH_1 204 shown in FIG. 3. The reserved channel isnot subject to contentions with high probability because the likelihoodof two or more mobile stations accessing the system with the same ID isvery small. In addition, in one embodiment, the reserved channel isassociated with a forward link power control channel (F-PCCH), such asF-PCCH_1 206 shown in FIG. 3, which provides closed loop power controlfor the mobile station as explained below. In one embodiment, based uponthe assignment of the R-RACH_1, the mobile station can determine theassociated F-PCCH. In another embodiment, the channel assignment messagespecifies both a R-RACH and a F-PCCH.

[0041] In one embodiment, the channel assignment message can specify await period. In this embodiment, the base station determines that acertain R-RACH which is currently in use will be available at some timein the future. It may make this determination based upon the knownlength of a message already in progress or based upon a known maximumlength for messages. In essence, the time delayed channel assignmentmessage tells the mobile station to begin transmission on the specifiedR-RACH after a predetermined number of frames have passed. This type ofoperation has the advantage of freeing the R-CCCH for use by othermobile stations, thus, decreasing the number of collisions andincreasing the overall efficiency of the system.

[0042] In block 154, the mobile station transmits a message portion 214of the access probe on the assigned reverse reserved access channelR-RACH_1 204 and receives power control commands 216 on the associatedF-PCCH_1 206 as explained more fully below. The message portion cancomprise a response to a page, an original request for a trafficchannel, a datagram bearing user information in a digital data system,or other type of message. In block 156, the mobile station has completedthe access attempt and the access routine enters an idle state.

[0043] Returning again to block 126, if the D1 timer expires before amatching hash ID is detected in a correctly received response message,flow continues to block 128. In block 128, the probe count isincremented. Block 130 determines whether the probe count is less than athreshold. If so, the maximum number of access probes have not been sentand flow continues to block 144 in which mobile station generates arandom number PN(p) for the back-off period. In block 142, the flowwaits the prescribed number of time slots designated by PN(b). In block140, the mobile station increases its transmit power and flow continuesback to block 118 where the access probe is transmitted at the higherpower level over the R-CCCH(c).

[0044] If it is determined in block 130 that the maximum number ofaccess probes have already been sent over the previously chosen R-CCCH,flow continues from block 130 to block 132. In block 132, the sequencenumber is incremented. Block 134 determines whether the sequence numberis less than a prescribe threshold. If so flow continues through offpage connector 138 back to FIG. 2A where, after a random delay, themobile station randomly selects a new F-CACH and R-CCCH pair over whichto attempt access to the system. If it is determined in block 134 thatthe sequence number is greater than or equal to the maximum sequencenumber, flow continues from block 134 to block 136 in which accessfailure is declared and the mobile station enters a system determinationstate.

[0045] The operation just described has a number of advantages inrelation to the access scheme defined in IS-95. The request portion ofthe access probe is transmitted over a slotted aloha channel in asimilar manner as the access probe in IS-95. However, according toIS-95, the mobile station transmits an entire access probe comprising alengthy ESN and message which may have a duration as long as 520 msec.According to IS-95, the mobile station then monitors a paging channelfor as much as 1360 msec for a traffic channel assignment message fromthe base station. If the traffic channel assignment message is notreceived, the mobile station sends the entire access probe again afterinsertion of a back-off period which can be as long as 8320 msec. Thus,in the event of a failure, as much as 9680 msec passes before the mobilestation retransmits the entire access probe, typically at a higher powerlevel than before, spewing even more energy into the system.

[0046] Thus, according to IS-95, typically 150 msec or more of energy istransmitted over the reverse link access channel whether or not the basestation can detect the signal. In this way, significant energy is expendon futile access attempts lowering the efficiency of the mobile stationpower consumption and creating useless interference to the system. Inaddition, this type of operation introduces a significant delay in theevent of an initial failure. The invention overcomes these limitations.

[0047] Under IS-95, the base station does not establish a forward linkconnection to the mobile station until the entire access probe has beenreceived. Therefore, the base station has no way of transferring powercontrol information to the mobile station during the transmission of thelengthy access probe. Without any power control, both the likelihood ofexcessive power generation (due to a transmission power level which istoo high) and the likelihood of repeat transmission (due to atransmission power level which is too low) are increased, thus,increasing the level of interference to the system. In one embodiment,the invention also overcomes this limitation by providing closed looppower control for the message portion of the access probe.

[0048] According to well known, acquisition techniques, detection of themobile station signal by the base station requires only a very smallfraction of energy transmitted in the prior art access probe. Therefore,in contrast, the present invention uses the request portion of theaccess probe in order to facilitate detection of the mobile stationsignal by the base station. The request portion of the access probe issignificantly shorter than the access probe in IS-95. For example, inone embodiment, the entire request portion can be transmitted in 2.5msec. Typically, the ratio of the duration of the request portion to theduration of the message portion is very small such as on the order of0.01.

[0049] After transmission of the brief request portion, the mobilestation ceases to transmit. If the base station receives the request, itresponds with the brief channel assignment message. Again, the messagemay be relatively short as it specifies the hash ID rather than theentire ESN. For example, in one embodiment, the reserved access channelassignment message is 3.75 msec in length. In this way, transmission ofthe reserved access channel assignment message does not consumesignificant system resources. And, in this way, the mobile station isinformed rather quickly as to whether the base station was able todetect its signal. For example, in FIG. 3, if the response message 212is a channel assignment message for the mobile station, the mobilestation is aware that the base station detected its signal approximately5 msec after the end of the transmission of the request portion. Thisentire transaction can take place in about {fraction (1/20)} of the timenecessary to just transmit an access probe according to IS-95.

[0050] Due to the short duration of the request portion of the accessprobe, the slot boundaries upon which the mobile station is permitted tobegin transmission according to slotted aloha operation can followclosely one after another. In this way, the number of possibletransmission times is increased which reduces the probability ofcollision and allows for more mobile stations to be supported by therandom access channel. For example, according to IS-95, the slotboundaries occur at a rate of 1.92 to 12.5 boundaries per second. In oneembodiment, the slot boundaries of the invention occur at a rate on theorder of 800 boundaries per second. If two mobile stations transmitduring the same slot boundary but the base station is able to detect oneor both of the requests due to diversity such as time diversity due topath delays, the base station may assign each contending mobile stationto a different R-RACH by reference to the hash ID, thus allowing thesystem to capture contending mobile stations in some situations.

[0051] If a failure does occur, the mobile station is aware of thefailure within the period D1 which is, in one embodiment, on the orderof 40 to 60 msec. The mobile station can send a follow up requestportion on one of the rapidly occurring slot boundaries which follows,thus reducing the delay introduced by a failure. In addition, due to thebrevity of the request portion, the amount of energy spewed uselesslyinto the system is greatly reduced in comparison with IS-95.

[0052] Once the mobile station is assigned a reserved access channel,the traffic channel assignment process can proceed in much the samemanner as IS-95. In addition to the message portion which specifies theresources requested by the mobile station, the mobile station alsotransmits a short preamble in the message portion of the access probe sothat the base station can detect the signal and perform coherentdemodulation. In one embodiment, the preamble in the message portion isabout 1.25 msec long.

[0053] One significant advantage of the use of the reserved multipleaccess scheme is that a forward link connection from the base station tothe mobile station is readily established in parallel with the reverselink reserved multiple access channel. In contrast, according tooperation under IS-95, the base station does not fully detect the mobilestation until the entire access probe has been received and the mobilestation does not begin to monitor for forward link signals until theentire access probe has been transmitted. However, in accordance withthe invention, the base station is aware of the mobile station aftertransmission of the request portion. The assignment of the R-RACH allowsa parallel forward link connection to the mobile station to be readilyestablished. The base station can monitor the R-RACH assigned to themobile station in order to quickly detect any transmission made by themobile station.

[0054] As noted above, in one embodiment, the system uses a parallelforward link channel to implement closed loop power control of themobile station transmission power during transmission of the messageportion of the access probe. Closed loop power control refers to controlof the mobile station transmission power by the base station. The basestation determines the proper transmission level based upon the actualoperating conditions at the base station. As shown in FIG. 3, in oneembodiment, a single F-PCCH is associated with a plurality of R-RACH's.The power control commands for multiple mobile stations are timemultiplexed onto the channel in a predetermined manner such that when amobile station is assigned to a R-RACH, it can determine whichinformation on the F-PCCH corresponds to its own transmission. In analternative embodiment, the power control packets can be interleavedwith data on a separate channel such as in a similar manner to thetraffic channel operation according to IS-95. In one embodiment, thepower control rate is programmable. For example, power control commandsmay be passed to the mobile station at 0, 200, 400 or 800commands/second. The power control rate may depend on the length of themessage as well as other factors such as system loading. A rate of 0commands/second may be used if the message is so short that the powercontrol won't take effect until after the message has ended.

[0055] Referring now to FIG. 4, an exemplifying structure of a stream ofpower control information packets 250 is shown. Each power controlinformation packet 250 is capable of carrying N power control commands252A-252N. In this way, N different R-RACH can be associated with asingle F-PCCH. In the embodiment shown in FIG. 4, each power controlcommand 252 in the power control information packet 250 maps to a singleR-RACH and is used to control the output power of the mobile stationcommunicating over that R-RACH. Thus, the power control command 252Acontrols the output power level of the mobile station transmitting onR-RACH_(—)1, the power control command 252B controls the output power ofthe mobile station transmitting on R-RACH 2, and so on. As noted above,in one embodiment, the system allows for variable rate power control,such that some of the power control information packets 250 can comprisemore than one command intended for a single mobile station or the F-PCCHcan control more than N R-RACH by time multiplexing power controlcommands in successive power control information packets. In such acase, the mapping of the power control information packets to theassociated R-RACH becomes less uniform but operates under the sameprinciples.

[0056] In one embodiment, the power control commands are a single bit inlength and the mobile station either raises or lowers its transmit powerin accordance with the single bit value in a similar manner as trafficchannel in IS-95. When a mobile station begins to transmit on aparticular R-RACH, the mobile station begins to monitor the powercontrol bit stream 250 and, in particular, to the power control command252 that is mapped to the particular R-RACH.

[0057] Referring now to FIG. 5, there is shown a timing diagramillustrating the power transmitted by a mobile station on a R-RACHaccording to the power control information commands received over theF-PCCH. At the beginning of the access channel time slot, the mobilestation transmits a preamble portion of the message portion of theaccess probe at an initial power level. Typically, the base station mustacquire the mobile station signal and accumulate a series of signalquality indications before it begins to send power control bits to themobile station. This delay is shown on both FIGS. 3 and 5 as D3. Theremainder of FIG. 5 shows an exemplifying sequence of mobile stationoutput powers in response to a series of power control commands receivedfrom the base station.

[0058] In one embodiment, the power control on the R-RACH is similar tothe power control on the traffic channel as described IS-95. Morespecifically, the base station can compare the power level of thereceived signal to a threshold. If the received signal is below thethreshold, the base station uses the power control information packet tosend a single bit power-up command to the mobile station. Otherwise, thebase station uses the power control information packet to send a singlebit power-down command to the mobile station. In one embodiment, each ofthe power control bits is modulated with BPSK modulation and can,therefore, assume one of three states, namely off, 0 degrees and 180degrees. More information concerning power control can be found in IS-95and in U.S. Pat. Nos. 5,056,109 and 5,265,119, both of which areentitled METHOD AND APPARATUS FOR CONTROLLING TRANSMISSION POWER IN ACDMA CELLULAR TELEPHONE SYSTEM and assigned to the assignee of thepresent invention and incorporated by reference herein in theirentirety.

[0059] Such closed loop power control is important for maximizingcapacity of a mobile radio telephone system according to well knowncommunications theories. Closed loop power control permits a mobilestation which begins a R-RACH access by transmitting its signal withmore power than is needed to be rapidly corrected to the desired powerlevel once the base station has acquired the mobile station'stransmission, thus reducing unnecessary interference in the system.Closed loop power control permits a mobile station which begins a R-RACHaccess by transmitting its signal with less power than is needed to berapidly corrected to the desired power level once the base station hasacquired the mobile station's transmission, thus reducing theprobability of failure.

[0060] The separation of the message portion as well as the provisionfor power control during transmission of the message portion also lendsflexibility to the system. For example, in a wireless data system, themobile station is likely to generate short bursts of data interspersedbetween significantly longer periods of idleness. Rather thanestablishing a traffic channel each time that the mobile station has aburst of data, it may be advantageous to use the access process justdescribed to bear user data. For example, the message portion of theaccess probe may contain a datagram of bearer traffic.

[0061] The invention lends itself particularly well to transmission ofdatagrams for several reasons. According to IS-95A, only a single datarate, 4800 bits/sec, is available for transmission of the access probe.According to the invention, the system can support a variety of datarates during access mode. In general, increased data rates are allowedif the mobile station can increase its transmission power so that theenergy devoted to each bit (Eb) remains fairly constant even if theduration of each bit is reduced. For example, in one embodiment, themobile station can increase its data rate to 9600 bit/sec, 19.2kilobits/sec or 38.4 kilobits/sec if sufficient transmission power isavailable. The use of higher data rates allows the mobile station totransfer messages faster than at the lower data rates so that theyconsume the channel for less time and reduce congestion in the system.The use of higher data rates also decreases the time delay associatedwith the transfer of large datagrams. The use of higher data rates ispractical because the closed loop power control which operates on theR-RACH allows the mobile station to increase its transmit power only tothe extent it is necessary.

[0062] In addition, the use of a reserved channel allows load control ofthe system. Load control is more intelligent than simple persistencebecause it takes into account the data rate of the incoming signal. If areserved channel carries data at an increased rate, it also consumes amore significant portion of system capacity. In one embodiment, themobile station includes an indication of desired data rate in thepreamble of the request portion. In another embodiment, the mobilestation can include an indication of desired data rate in the preambleof the message portion. In yet another embodiment, the base stationdetermines the data rate by reference to the implicit features of themobile station signals. The base station uses the data rate to determinecurrent system loading. If system loading reaches a predeterminedthreshold, the base station can, for example, begin to send waitmessages to specific or all requesting mobile stations or can directspecific or all mobile stations to use a specified data rate.

[0063] In one embodiment of the invention, the system incorporatespseudo softer handoff operation on the forward link, on the reverse linkor both. FIG. 6 is a representational diagram showing the coverage areasectors of a multisectored base station. A multisectored base station270 transmits signals into three different sector coverage areas272A-272C. The sector coverage areas 272A-272C overlap to some extent incoverage overlap areas 274A-274C to provide a continuous coverage areaassociated with the base station. Within the coverage overlap areas274A-274C, the system signal levels are sufficient for the mobilestation to establish bi-directional communication with the base stationthrough the two intersecting sectors. Such operation is detailed in U.S.Pat. No. 5,625,876, entitled METHOD AND APPARATUS FOR PERFORMING HANDOFFBETWEEN SECTORS OF A COMMON BASE STATION, assigned to the assigneehereof and incorporated herein in its entirety by this reference.

[0064]FIG. 7 is a block diagram of the multisectored base station 270.Antennas 280A-280C receive signals from sector coverage areas 272A-272C,respectively. In one embodiment, one or more of the antennas 280A-280Care diversity antennas comprising two or more separate antenna elements.The antennas 280A-280C provide received energy to radio frequency (RF)processing blocks 282A-282C, respectively. The RF processing blocks282A-282C down-convert and quantize the received signal energy toproduce digital samples using any one of a myriad of well knowntechniques.

[0065] Demodulators 284A-284C receive the digital samples and demodulateone or more reverse link signals contained therein. In one embodiment,the demodulators 284A-284C comprise a set of demodulator elements andsearcher elements such as those disclosed in U.S. Pat. No. 5,654,979,entitled CELL SITE DEMODULATION ARCHITECTURE FOR A SPREAD SPECTRUMMULTIPLE ACCESS COMMUNICATION SYSTEMS, assigned to the assignee hereofand incorporated herein in its entirety by this reference. According tothe '979 patent, each demodulator comprises a set of demodulationelements, each of which can be assigned to a multipath propagation ofone of the reverse link signals. The outputs of the demodulationelements are combined to create a resultant signal.

[0066] If a mobile station is in softer handoff, two or more of thedemodulators 284 are assigned to demodulate the same reverse linktraffic channel signal from the mobile station. The demodulators 284output demodulated signals to a signal combination block 288 which canfurther merge traffic channel signals received through more than onesector. The output of the signal combination block 288 is coupled to asignal processing unit 290 which performs further signal processing onthe merged output.

[0067] A signal generation block 292 creates the forward link signals.The signal generation unit 292 provides forward link signals to one ormore of the modulators 286A-286C depending on the location of the mobilestation. Only those sectors with established bi-directionalcommunication transmit a traffic channel to the mobile station, thusreducing interference in those sectors which do not service the mobilestation. The modulators 286A-286C modulate the signals for wireless linktransmission and pass them to the RF processing blocks 282A-282C,respectively. The RF processing blocks 282A-282C convert the digitalbits to analog signals and up-convert them to the desired transmissionfrequency. The antennas 280A-280C radiate the signals into thecorresponding coverage areas sectors 272A-272C.

[0068] According to the prior art, the softer handoff techniques areassociated only with the traffic channel where sustained bi-directionalcommunication is established between the base station and the mobilestation. According to IS-95, the access probes are only received by asingle sector of a multisectored base station regardless of whether themobile station is located in a coverage overlap area. Likewise,according to IS-95, the channel assignment message is transmitted fromonly one sector of a multisectored base station regardless of whetherthe mobile station is located in a coverage overlap area.

[0069] In general, each R-CCCH is associated with just one sector and arequest portion of an access probe is detected by only one sector. Inone embodiment of the invention, the base station 270 is configured tobroadcast the F-CACH in all sectors of the base station in a so-calledsimulcast mode. In this way, a mobile station located within a coverageoverlap area transmits the request message 210 to one sector but canreceive the response message 212 from more than one sector, thus,increasing the combined signal energy detected by the mobile station andincrease the probability of successful reception by the mobile station.This type of pseudo softer hand off operation during the access processmimics softer handoff on the forward link traffic channel. Thus, in FIG.7, the signal generation block 292 creates the F-CACH and passes it toeach of the modulators 286A-286C regardless of the origin of the requestportion for which the response messages are generated. These sameprinciples can be applied to transmission of the F-PCCH from multiplesectors. In another embodiment, the reliability of the reception of theF-CACH and F-PCCH by the mobile station is improved within a sector byusing transmit diversity. In this embodiment, replicas of the sameinformation are transmitted on different antenna elements within a givensector, using one or more diversity techniques such as orthogonal codediversity, time division repeated transmission, and delay transmissions.

[0070] In a similar manner, this principle can be extended to other basestations operating in the same area. Thus, when a mobile station sends arequest portion of an access probe, a set of base stations in a zonesurrounding the detecting base station respond with transmission of theresponse message. These same principles can be applied to transmissionof the F-PCCH from multiple base stations. This type of pseudo soft handoff operation during the access process mimics soft handoff on theforward link traffic channel.

[0071] As noted above, according to IS-95, the base station does notfully detect the mobile station signal until the entire, rather lengthy,access probe is received by the base station. Thus, according to IS-95,the softer handoff techniques applied to the traffic channel cannot beapplied to the access process because the sector to which the accessprobe is directed cannot identify the signal to the other sectors sothat they may also detect the signal. In contrast, according to theinvention, the majority of the access probe is transmitted over theeasily identifiable R-RACH. Thus, in one embodiment, a plurality ofsectors demodulate the R-RACH and provide corresponding signal energyoutputs. For example, when the request portion 210 is received over aR-CCCH associated with coverage area sector 272A, each of thedemodulators 284A-284C attempt to demodulate the R-RACH assigned to themobile station. In this way, if the mobile station is located within oneof the coverage overlap areas 274A-274C, the message portion of theaccess probe is received by each of the corresponding sector'sdemodulators 284. The resultant signals are merged by the signalcommunication block 288 and a single power control indication based uponthe combined signal is generated. As noted above, the power controlindication can be transmitted from more than one sector over a simulcastF-PCCH. This type of pseudo softer hand off operation during the accessprocess mimics softer handoff on the reverse link traffic channel.

[0072] In a similar manner, this principle can be extended to other basestations operating in the same area. Thus, when a mobile station sends arequest portion of an access probe, a set of base stations in a zonesurrounding the detecting base station attempt to demodulate the R-RACH.This type of pseudo soft hand off operation during the access processmimics soft handoff on the reverse link traffic channel.

[0073] Incorporation of pseudo softer handoff, pseudo soft handoff orboth on the reverse link greatly facilitates the proper operation of thepower control on the R-RACH. Unless each base station and sector whichis capable of receiving the mobile station signal at a significant levelis able to contribute to the power control commands sent to the mobilestation, the mobile station signal strength can become excessive at thenon-contributing base stations and jam communications therethrough.Therefore, in one embodiment, each surrounding base station and sectorattempts to demodulate the signal from the mobile station on the R-RACHand contributes to the power control command sent to the mobile station.

[0074]FIG. 8 is a block diagram of an exemplifying mobile stationarchitecture. An antenna 302 receives and transmits signals over awireless link to a base station. An RF signal processing block 304 iscoupled to the antenna 302. The RF signal processing block 304down-converts and quantizes the received signal energy to producedigital samples using any one of a myriad of well known techniques. TheRF signal processing block 304 is coupled to a modulator/demodulator(modem) 306. The modem 306 receives the quantized energy and demodulatesthe incoming signal under the control of a control 308. In oneembodiment, the modem 306 operates in accordance with U.S. Pat. No.5,764,687, MOBILE DEMODULATOR ARCHITECTURE FOR A SPREAD SPECTRUMMULTIPLE ACCESS COMMUNICATION SYSTEM, which is assigned to the assigneehereof and incorporated herein in its entirety by this reference. Themodem 306 also modulates signals for transmission over the wireless linkunder the control of the controller 308. The modulated signals arecoupled to the RF signal processing block 304 which converts the digitalbits to analog signals and up-converts them to the desired transmissionfrequency for transmission over the antenna 302. In one embodiment, theblocks shown in FIGS. 2A and 2B are carried out by a series ofprocessing units stored in a memory 310 and executed by the controller308. In one embodiment, the mobile station comprises an applicationspecific integrated (ASIC) circuit for execution of the functions. Inanother embodiment, the process blocks are stored in a programmablestorage device.

[0075] Although the invention has been described in the context of aCDMA system where some of the CDMA channels are further channelizedusing time division techniques, other channelization techniques canbenefit from the general principles described herein. For example, timedivision multiple access (TDMA) and frequency division multiple access(FDMA) channels could be used in accordance with the principles of theinvention. In addition, the messages on the channels can be coded andinterleaved. The messages can be repeated and the energies combined toimprove reliability. Quadrature techniques can be used to increase therate at which data is carried over the channels.

[0076] Other alternative embodiments will be readily apparent to oneskilled in the art upon examination of the principles discussed hereinincluding the simple re-arrangement of the blocks shown in FIGS. 2A and2B. For example, the advantages gained by reducing the size of themobile station identification transmitted in the request portion may begained by reduction in size in other manners aside from the use of ahash function. In one embodiment, the mobile station may randomly choosea quasi-unique identification as a temporary identifier of the mobilestation. In one alternative embodiment, once the mobile station sendsthe request portion of the access probe, it monitors the pilot signalstrength as well as the F-CACH. If the pilot signal strength isrelatively high but the F-CACH does not carry a response message, themobile station determines that the base station did not detect therequest portion because the signal level was too low. Therefore, themobile station, without inserting arbitrary delay, retransmits therequest portion at a higher signal level.

[0077] In one embodiment, the base station periodically sends abroadcast access control message. The access control message is used bythe mobile station to determine the loading conditions of the system.The access control message comprises a message type field containing avalue which indicates that the message is an access control messageintended for reception by all mobile stations. The access controlmessage also comprises a persistence parameter field containing a valuewhich is used by the mobile station to determine the back-off timervalue. The access control message also comprises a minimum wait timefield containing a value which indicates the minimum value to be used inthe persistence test, for load/flow control. If the minimum wait timefield is set to its maximum value, accesses are shut off. Other systemconfiguration information and related parameters can be carried on aforward link common control channel such as the paging channel in IS-95.

[0078] In another embodiment, the mobile station transmits a pilotsub-channel along with the message portion of the access probe. Theinclusion of the pilot sub-channel may be performed by any one of amyriad of well known techniques. The sub-channel can be used by themobile station to provide power control information to the base stationconcerning the power level at which it is receiving the F-PCCH. That is,the mobile station uses a small fraction of the pilot channel to conveyincrease or decrease commands to the base station so that the powerallocated to its F-PCCH sub-channel is adjusted to the minimumacceptable level in order to conserve system resources.

[0079] In yet another embodiment, if the mobile station has a shortmessage to transfer, it sends a request message on the R-CCCH with thehash ID set to all 0's (or some other pre-selected value) whichindicates to the base station that additional data follows immediatelyand that no channel assignment is required. The data that follows istransferred, for example, within about 5 msec, and, therefore, is tooshort to realize any significant benefit from using closed loop powercontrol. In such a case, it can be more efficient to communicate thisinformation on the random access channel rather than wait for theassignment of a reserved access channel. The request message is notsubject to power control because it is being transmitted on the R-CCCH.

[0080] In one embodiment, the channel assignment message has a 1 bitindication which is used to inform the accessing mobile station that thebase station has received multiple request portion messages in the sameaccess slot. In this way, a mobile station awaiting a response on theF-CACH more quickly determines whether it should resend the requestportion at a higher power level or the same power level or continue towait for an assignment message. This feature can be used to reducetransmission delay.

[0081] In another embodiment, the channel assignment message may containa power control correction value which is used by the mobile station toadjust its transmit power prior to closed loop power control beingenabled on the reserved channel. In this scheme, the base stationdetermines the adjustment necessary to support reliable communicationsbased on, for example, the requested or assigned data rate as well asthe received energy detected over the request portion of the mobilestation's transmission.

[0082] In still a further embodiment, a class wait message is used toeffect the behavior of a class of mobile stations attempting to accessthe system. A class wait message indicates that those mobile stationswhich have a class mark less than or equal to a class mark threshold areforced to use a different set of persistence and back-off parameters orto cease attempting to access the system and revert back to monitoringthe appropriate overhead channel to get updated access parameters. Thosemobile stations which have a class mark greater than the class markthreshold are permitted to continue accessing the system, either usingexisting or updated persistence and back-off parameters. In this way,the system has a mechanism for quickly disabling accesses in aprioritized manner in order to control loading.

[0083] In yet another embodiment, mobile stations wishing to access thesystem can monitor the activity on the F-PCCH, F-CACH or both in orderto derive an estimate of system loading. This estimate can be used toaffect the parameters that affect the access behavior of the mobilestation, such as persistence, back-off, data rate, and such. This schemecan be used effectively to increase the efficiency of the requestchannel in certain operating environments.

[0084] In one embodiment, the invention is embodied in a system whichuses a set of binary code sequences as signatures. For example, thepreamble sent by the mobile station to the base station to access thesystem is one of a set of predetermined, distinguishable code sequencescalled signatures. The mobile station selects one of the signatures totransmit each time it attempts to access the system. For example, themobile station randomly selects one of 16 different 1 msec longsignatures and transmits it during one of a series of 1.25 msec timeslots. Or, the mobile station can generate a signature based upon themobile station's uniquely identifying number.

[0085] The base station monitors the R-CCCH for each of the 16signatures in all time slots. When the base station detects a signature,it responds to the mobile station on the F-CACH with a message whichreflects the signature used by the mobile station. For example, in oneembodiment, the base station responds to the mobile station on theF-CACH using a message modulated with the same binary code sequence usedby the mobile station. In another embodiment, the base station respondswith a message modulated by a different binary code sequence which isassociated with the signature used by the mobile station in apredetermined fashion. In another embodiment, the base station respondson the F-CACH by including a field which designates the signature usedby the mobile station. For example, if there are 16 signaturesavailable, the base station can specify which signature of the accessprobe to which it is responding by a field with four bits. In this way,the overhead burden of transmitting the hash function is eliminated onboth the F-CACH and the R-CCCH, thus decreasing the amount of systemresources expended on these tasks.

[0086] In one embodiment, each signature comprises a 256 bit sequencewhich is repeated 16 times and which is modulated according to 16 bitmask. On the reverse link, the sequence can be a segment of a Gold codesuch as those describe on pages 833 and 834 of John Proakis, DigitalCommunications, Second Edition, McGraw-Hill Book Company (1989). On theforward link, the sequence can be an orthogonal variable spreadingfactor (OVSF) code or variable length or hierachical Walsh code oflength 256 as are well known in the art. In this way, the forward linktransmission on the F-CACH are orthogonal to the other forward linktransmissions.

[0087] In one embodiment, one or more of the binary code sequences usedby the base station on the F-CACH are reserved to indicate a level ofloading at the base station. For example, one of the binary codesequences can indicate that a maximum loading has been exceeded, thusindicating to the mobile station to wait until the load indicator isturned off, to enter a back-off procedure according to persistenceparameters or both. In this embodiment, after transmitting a signatureover the R-CCCH, the mobile station monitors the F-CACH in order todetect the responsive message corresponding to the transmitted signatureas well as one or more of the signatures which indicate the level ofloading.

[0088] In one embodiment, the polarity of the binary code sequencetransmitted by the base station on the F-CACH conveys information to themobile station. For example, the polarity of the code sequence can beused to convey power control information to the mobile station. Onepolarity can indicate to the mobile station to increase the level atwhich it transmits on the R-RACH above the level used on the R-CCCH andthe inverse polarity can indicate to the mobile station to decrease thelevel at which it transmits on the R-RACH below the level used on theR-CCCH. The polarity could also be used to set or limit the data rate atwhich the mobile station transmits on the R-RACH.

[0089] The use of signatures can be characterized as the use of alimited number of semi-unique ID's (such as the hash ID's) used tomodulate the signal. In one embodiment, a randomly chosen signature ismodulated according to a hash ID associated with the mobile station. Inanother embodiment, the hash ID is carried as data in a messagemodulated with the signature. When the base station responds on theF-CACH, the response message can indicate the hash ID. The use of a hashID reduces the probability of false capture if two mobile stations usethe same signature at the same time but the base station detects onlyone. If the mobile station whose signal was not detected decodes themessage on the R-RACH comprising the hash ID of the other mobilestation, it can thus determine that the R-RACH transmission is intendedfor another mobile station.

[0090] In one embodiment, the hash ID is transferred implicitly over theF-CACH. For example, the binary code sequence used to modulate thechannel assignment message is a unique function of the hash ID. Thus,the hash ID is not sent explicitly along with the other data intendedfor the mobile, but instead is used to select or modify the code used onthe F-CACH. The mobile station demodulates the F-CACH using the codesequence derived based on its own hash ID and, if it successfullydecodes the acknowledgement, can proceed to transmit the remainder ofthe message on the assigned R-RACH channel. In one embodiment, aseparate forward link channel which is orthogonal to all other forwardlink channels is used to convey the hash ID and other data. In anotherembodiment, the hash ID is used to modify the existing sequence used onthe forward link channel used to convey the hash ID and other data. Inthis way, the binary code sequence used on the F-CACH can be function ofthe signature, the hash ID or both.

[0091] The invention may be embodied in other specific forms withoutdeparting from its spirit or essential characteristics. The describedembodiment is to be considered in all respects only as illustrative andnot restrictive and the scope of the invention is, therefore, indicatedby the appended claims rather than the foregoing description. Allchanges which come within the meaning and range of equivalency of theclaims are to be embraced within their scope.

What is claimed is:
 1. A method for accessing a base station from amobile station in a communication system, comprising: selecting asignature sequence from a plurality of signature sequences; modulatingsaid selected signature sequence with a request portion of an accessprobe; transmitting said modulated request portion over a reverse linkcommon control channel; receiving a channel assignment message from saidbase station designating a reserved access channel, said reserved accesschannel providing communication with a low probability of contention andsaid channel assignment message reflecting said selected signature;transmitting a message portion of said access probe over said reservedaccess channel; receiving a power control command over a forward linkchannel associated with said reserved access channel; and responding tosaid power control command by increasing or decreasing a power level atwhich said mobile station transmits said message portion.
 2. The methodof claim 1, wherein said transmitting said message portion occurs at oneof a plurality of available data rates.
 3. An apparatus for accessing abase station from a mobile station in a communication system,comprising: means for selecting a signature sequence from a plurality ofsignature sequences; a modulator for modulating said selected signaturesequence with a request portion of an access probe; a transmitter fortransmitting said modulated request portion over a reverse link commoncontrol channel; a receiver for receiving a channel assignment messagefrom said base station designating a reserved access channel, saidreserved access channel providing communication with a low probabilityof contention and said channel assignment message reflecting saidselected signature; said transmitter further for transmitting a messageportion of said access probe over said reserved access channel; saidreceiver further for receiving a power control command over a forwardlink channel associated with said reserved access channel; and means forresponding to said power control command by increasing or decreasing apower level at which said mobile station transmits said message portion.4. The apparatus of claim 3, wherein said transmitter further fortransmitting said message portion at one of a plurality of availabledata rates.
 5. A method for accessing a base station from a mobilestation in a communication system, comprising: receiving a modulatedrequest portion over a reverse link common control channel, whereinmodulated request portion is modulated with a selected signaturesequence; transmitting a channel assignment message from said basestation designating a reserved access channel, said reserved accesschannel providing communication with a low probability of contention andsaid channel assignment message reflecting said selected signature;receiving a message portion of said access probe over said reservedaccess channel; transmitting a power control command over a forward linkchannel associated with said reserved access channel for increasing ordecreasing a power level at which said mobile station transmits saidmessage portion.
 6. The method of claim 5, wherein said receiving saidmessage portion occurs at one of a plurality of available data rates. 7.An apparatus for accessing a base station from a mobile station in acommunication system, comprising: a receiver for receiving a modulatedrequest portion over a reverse link common control channel, whereinmodulated request portion is modulated with a selected signaturesequence; a transmitter for transmitting a channel assignment messagefrom said base station designating a reserved access channel, saidreserved access channel providing communication with a low probabilityof contention and said channel assignment message reflecting saidselected signature; said receiver further for receiving a messageportion of said access probe over said reserved access channel; saidtransmitter further for transmitting a power control command over aforward link channel associated with said reserved access channel forincreasing or decreasing a power level at which said mobile stationtransmits said message portion.
 8. The apparatus of claim 5, whereinsaid receiver further for receiving said message portion at one of aplurality of available data rates.